When gamma cameras of the Anger type were first placed in use as medical diagnostic tools, they were used to produce images of organs such as the brain and the thyroid gland. After significant improvements had been made in the cameras and new radioactive isotopes were developed, the cameras commenced to be used for conducting so-called whole body studies for such purposes as detecting cancer of the bone marrow.
Emission computed tomography (ECT) is now used clinically to reconstruct images in a manner which can be likened to its more famous x-ray counterpart, computed tomography, or CT. At least from the standpoint of achieving status as a routine examination procedure, ECT is a more recent development than are whole body studies but utilization of the procedure is increasing.
With an ECT study a nuclear camera detector is orbited about the patient. At each selected point in the orbit, output signals are developed by the detector and its decoding electronics in a conventional manner. The output signals are fed to a computer and after data from a sufficient number of angles or points have been collected, the computer supplies information, typically to an oscilloscope, where an image of a transverse slice through the patient is displayed.
In one commercial system a stand is provided which has a through hole of sufficient size to permit a cantilever supported patient stretcher and a supine patient to pass at least part way through it. The stand assembly carries a rotatable ring which surrounds the hole. A generally rectangular yoke is pivotably supported on the ring for tilting motion about an axis which is perpendicular to and intersects the ring axis. A camera detector is pivotally mounted near one end of the yoke for pivotal adjustment motion about an axis paralleling the yoke tilt axis. A detector counterweight is fixed to the yoke near its end opposite the detector
When the described arrangement is in use, a patient is positioned on a cantilever supported and movable stretcher. The yoke is pivoted to position the detector a desired distance from the patient and then fixed in its adjusted position. The detector is moved about its pivot to bring its face into a plane paralleling the through hole axis which is also the axis of orbital rotation for ECT studies. The detector is then incrementally or continuously orbited about the patient and an ECT study is made.
Alternately the detector is maintained in a stationary position, as is the patient, for a still or so-called spot image. As a still further alternate the detector is maintained in a stationary position while the patient and stretcher are moved into and at least part way into the through hole to conduct what the manufacturer claims to be a whole body study.
The described commercial device has a number of disadvantages. One is that a change of collimators is rather difficult to accomplish. It is necessary to put a prop under the counterweight to support the yoke before removing a collimator and replacing it with another collimator. It is also necessary to be certain that the collimators available for use on the camera all are substantially equal in weight to maintain the counter-balanced condition which is needed to perform the described diagnostic studies.
Another disadvantage of this machine is that the described whole body and ECT studies can only be accomplished on a patient after he has been transferred from his bed to a mobile stretcher and thence to the patient table of the detector system.
In another commercial system, a gantry is provided that has a ring, and defines a patient aperture in its middle. The ring carries one or two U-shaped yokes. The yokes are pivotally mounted on the ring and each carries a nuclear detector at its closed end. At the open end of the U, a pair of spaced counterweights are provided. A stretcher is cantilever supported on a side of the yoke opposite the detectors.
When the latter machine is in use, the patient is positioned on the stretcher and the stretcher is then moved axially through the gantry aperture to position the patient relative to the detector or detectors. With this machine anterior and posterior spot images can be concurrently produced or both detectors may be used for ECT studies. The manufacturer also claims a capability of conducting whole body scans. Again, whole body scans, to the extent they may be achievable, require that a hospitalized patient be moved from his bed to a transporting stretcher and thence the machine stretcher if a whole body study is to be, and can be, conducted.